A window is an opening cut through a wall and given a closure. Shutters and boards seal it like a small door, and a grille of iron bars leaves it open to sight and air while keeping a body out — which is the difference that matters in play, because you can watch a room through bars and you cannot through timber. Windows admit daylight, offer a shot for anyone patient enough to wait at one, and give a climber a way into a room that has no door.